Nutrients are supplied to the mixed layer of the open ocean by either
atmospheric deposition or mixing from deeper waters, and these nutrients drive
nitrogen and carbon fixation. To evaluate the importance of atmospheric
deposition, we estimate marine nitrogen and carbon fixation from present-day
simulations of atmospheric deposition of nitrogen, phosphorus and iron. These are
compared with observed rates of marine nitrogen and carbon fixation. We find
that Fe deposition is more important than P deposition in supporting N fixation.
Estimated rates of atmospherically-supported carbon fixation are considerably
lower than rates of marine carbon fixation derived from remote sensing,
indicating the subsidiary role atmospheric deposition plays in total C uptake by
the oceans. Nonetheless, in high nutrient, low chlorophyll areas, the contribution
of atmospheric deposition of Fe to the surface ocean could account for about 50%
of C fixation. In marine areas typically thought to be N limited, potential C
fixation supported by atmospheric deposition of N is only ~1 ¿ 2% of observed
rates. Although these systems are N-limited, the amount of N supplied from
below appears to be much larger than that deposited from above. Atmospheric
deposition of Fe has the potential to augment atmospherically-supported rates of
C fixation in N limited areas. In these areas, atmospheric Fe relieves the Fe
limitation of diazotrophic organisms, thus contributing to the rate of N fixation.
The most important uncertainties in understanding the relative importance of
different atmospheric nutrients are poorly understood speciation and solubility of
Fe as well as the N:Fe ratio of diazotrophic organisms.